Westchester, Rockland and Putnam counties are filled with houses that were built more than 100 years ago. We also have a lot of houses that are, quite literally, being finished as you read this.

Whether you're considering buying a historic Colonial or a turn-key contemporary, homes from every era have their charms — and quirks — along with some features unique to each that need to be carefully considered before you buy. Materials like lead paint and asbestos, for example, were considered standard in the 1920s and '40s, but no homeowner wants them today.

As desirable as original features may be — like that expansive back deck on a 1970s' era split level you're considering — they should be looked at closely for aesthetic and safety reasons.

Starting in the 1920s and going decade by decade to the 2000s, we asked three experts for things that homeowners should look for in homes of each period: John D. Fry, AIA, of the American Institute of Architects New York State, Paul Coombes of Mr. Inside Out Home Inspections, and Bruce Elton, a licensed and bonded designer and home builder.

The 1920s:

Many home styles were popular in the 1920s including bungalows, Craftsman, neo-Tudor, neo-Colonial and the American up being that decade's main enemy. It was in the paint on the walls, and perhaps because covering the paint was more common than removing it, under wallpaper. Many of the main water supply pipes in homes built in this era were also lead.

Mortar and stone work: The mortar used in stone and brick foundations in homes built during this period might now be deteriorating and allowing water and moisture to seep into unfinished basements, which, if unfinished, can spawn mold and mildew. The home's interior walls are also vulnerable. Black mold is especially dangerous to your health.

Good to know: "You also have to think about the fireplaces in homes from the 1920s," says Fry. "They worked well back then because they tended to have good draw and the houses leaked air by default. But many of the windows in homes from that era have been replaced and are more air-tight. They could also have a new heating and air conditioning system with a return-air vent system. Those factors influence the natural draw of a fireplace and produce indoor smoke, so you need to create an air vent in back of the fireplace chamber to allow the intake of fresh air."

The 1930s:

The Cape Cod and Dutch Colonial were popular styles through the 1930s.

Attics: That unfinished attic space, commonly found in homes of this era, might look like a great opportunity for a master suite,but zoning codes now require that third floors have sprinkler systems that meet building and fire code regulation. Updated attics also now require an exit window that provides a clear opening and is a certain number of feet above the floor.

If an attic has its original window, it's probably sloped and doesn't meet those requirements. You might need to consider installing dormer windows, which are cost effective, attractive, and can accommodate safety-exit and zoning (bulk and height) requirements.

Insulation: Fine-particle material was used as insulation in the attics' ceilings; it wasn't very effective. Nor was the newspaper shoved between the space between the outside and inside walls.

Wiring: The knob and tube electrical wiring used in the 1930s' central wiring systems come up short compared to today's electrical demands. The oldies' 60- to 100-amps service could pose a fire hazard, so an upgrade to at least 200 amps is highly advisable.

Good to know: "The electrical system is the most important factor in the house," says Elton. "Insurance companies want them updated. Installing a whole new electric system and updating electric panels can be a significant expense."

The 1940s:

"This was the biggest single transition period in American home-building," says Fry. "We started seeing more of the Bauhaus design (marked by an emphasis on functional design)."

Asbestos: The infamous asbestos came to mass production. It was used in roofs, pipes, heat lines, attics, outside siding, flooring materials, ceiling tiles and wallboards. The particles have broken down by now and can pose health problems. In most cases, asbestos has already been addressed and dealt with in homes from that period. If it hasn't, you'll need to have it removed.

Oil tanks: Buried oil tanks were also common. If they haven't been replaced or removed, it's time for a new one that's above ground.

Good to know: "Bauhaus designed homes sported low-pitched and flat roofs that weren't well drained," says Fry. "A brand-new flat roof had a 15-year lifespan; flat 1930s roofs replaced in the 1960s had a 20- to 25-year lifespan. If they were again replaced in the 1980s, those roofs might need to be redone now. Today's flat-roof materials last a lot longer than before."

Homes from the 1950s had galvanized pipes and single pane windows.(Photo: John Fry)

The 1950s:

This was the decade of production housing, when newly built homes looked alike and were driven more by builders than architects and design. Long Island's Levittown, America's first mass-produced suburb, is the archetype for our postwar suburbs. These houses, many of them one-story ranches, weren't big on design details.

Galvanized pipes: Galvanized supply and waste pipes were mainstream. They corrode from the inside out, and the resulting clogs reduce the flow of entering and exiting water.

Single pane windows: "Homes were built with single-paned windows whose aluminum frames expanded, contracted, leaked and attracted the cold," says Coombes. "They were the biggest source of heat loss, and their seals have probably become ineffective." Replace them with double-paned windows.

The 1960s:

Though the designs and materials in homes from the 1960s tended to be extensions of the 1950s — ranches, asbestos, spackle, popcorn ceilings, flat roofs and large walls of pine paneling — there were some innovations, such as Formica counter tops (in 99 percent of the homes) and a penchant for track and florescent lighting. Some homes even had Cold War bomb shelters.

Floor joists: The two-by-six floor joists (the horizontal supporting members that run between foundations, walls or beams to support a ceiling or floor) of this era can't handle the weight of today's ceramic floors, granite counter tops and heavy appliances. Replace them.

Exterior stairs: This era's homes were also built with stairs up to the front door. "Those steps brought soil to the top of the foundation," says Coombes. "Termites headed under the stoop and attacked the wood underneath. Fifteen to 20 percent of those stairs have termite damage because no one ever checked them."

Looking at a bi-level from the '70s? Check out the back deck first.(Photo: Submitted photo)

The 1970s:

Hybrid houses — split-level ranches or colonials with one full floor and a half second floor — reached their apex. Expanding the upper level is popular, but recent building codes state that new handrails of stairs that run from the first floor to any upper floors must be continuous from beginning to end. Meanwhile, the balusters can't be more than four inches apart.

Boilers: Boilers from that era — some homes have the originals — aren't energy efficient.

Outdoor floodlights: Those from this era weren't all that light sensitive. Ditch the incandescent and go for the motion-sensitive LEDs.

Beware the back deck: Many houses were built with wooden back decks. "They probably had a 25-year lifespan before needing serious upgrading or replacement," says Fry. "Sometimes the horizontal walking surface or even the basic structural frame must be replaced. Composite materials are used today."

"Only nails — not the much stronger lag bolts — were used to attach the decks and steps to homes, and the wood wasn't pressure-treated. So many decks eventually collapsed," adds Coombes. "And because there were no safety codes for rails, many of them were low enough for kids to fall through and tumble over. Meanwhile, the handrails' spindles were horizontal instead of vertical, so kids used them as ladders, climbed up them, and then fell through or fell off at the tops of the handrails."

The 1980s

Houses were becoming more air-tight but didn't "breathe well."

Check the stucco: The early version of the exterior insulation finishing system known as EIFS, also called synthetic stucco, managed static dampness poorly, which resulted in mold and mildew. Natural stucco was more breathable and managed moisture. Also be aware of interior materials, such as formaldehyde, carpets or paints, that emit various detrimental vapors.

Modular homes entered the scene in the 1980s. "A lot of them weren't built well," says Coombes. "The manufacturing process was weak, the walls were thin and they were shoddily put together."

The 1990s:

Particle board: Oriental strand board, an engineered wood particle board formed by adding adhesives and then compressing layers of wood strands (flakes) in specific orientations, appears in homes of this era. The problems: It cracks around doors and windows, moisture gets behind it and it later rots and peels. If Oriental strand board hasn't been identified and repaired, you could have a problem on your hands.

Technology: Early versions of home technology started to appear, says Fry, "so security systems and home automation are probably severely outdated and may have to be replaced. Thermostat systems installed in the early 1990s were manually programmable, but thermostats are now controllable from your iPad; and lighting systems that turned lights on and off as a security element are a lot more sophisticated."

The 2000s and up:

Houses were built with the environment and energy efficiency in mind. Their roofs and paint finishes should still be good today.

Good to know: Not all brand-new homes are perfect, though, says Coombes. "There can be issues with contractors' bad craftsmanship and construction. It all comes down to dollars and cents — cutting corners to save money on framing, attic rafters and plumbing. They must get inspected."

Meet the experts

• John D. Fry: AIA, the president of the American Institute of Architects Westchester + Hudson Valley Chapter

• Paul Coombes: president of Mr. Inside Out Home Inspections, a licensed home inspection company based in New Windsor, N.Y., and does much of its work in Rockland and Westchester

• Bruce Elton: Real estate agent with North Country Sotheby's International Realty in Croton-on-Hudson, as well as a licensed and bonded designer and home builder who specializes in restorations.